Air temperature and relative humidity of an environment in which plants grow affect plant growth and health by affecting photosynthesis and transpiration. Photosynthesis is a process by which plants convert carbon dioxide and water to organic compounds needed for plant growth and metabolism. Transpiration is a process by which plants draw water and nutrients required for plant growth and metabolism from soil into their roots and transport the water and nutrients to their leaves and other plant organs.
Photosynthesis and transpiration are temperature and relative humidity dependent. Relative humidity, is a ratio equal to an amount of water contained in the atmosphere per unit volume of air divided by a maximum amount of water a unit volume of the air can contain before the water begins to condense out of the air. Water condenses out of air when the air's relative humidity is equal to 100%. Photosynthesis generally increases with increase in temperature. Transpiration is affected by a rate at which water drawn in from the soil and transported to plant leaves and organs evaporates from surfaces of the leaves and organs and increases with increase in rate of evaporation. Evaporation of water from plant surfaces also aids a plant in dissipating heat and regulating plant body temperature. Rate of evaporation and therefore transpiration, and a plants ability to cool itself, generally decreases with increasing relative humidity.
Plants adapted to different natural environments, for example, desert plants such as cactuses and tropical plants such as orchids, thrive in different temperature and relative humidity ranges. If they are subjected to temperatures and relative humidities outside of the ranges for which they are adapted, they generally do not do well, and may become diseased. Relative humidity in an environment in which a plant grows that is greater than a maximum for which the plant is adapted can result in a reduction in rate of evaporation to such an extent that concomitant reduction in plant transpiration, and the plant's ability to dissipate heat and regulate its body temperature, damages plant metabolism and health. High relative humidity also tends to result in condensation of water droplets on surfaces of plants when ambient temperature in the environment decreases during the diurnal cycle. The condensed moisture promotes germination of fungal pathogen spores, such as Botrytis and powdery mildew, on the plant surfaces that can damage or kill the plants.
Because of the sensitivity of plants to temperature and RH, artificial environments, such as provided by greenhouses, in which plants are commercially grown, must generally be monitored and controlled to maintain air temperature and humidity within desired ranges. For many greenhouse environments in which leafy plants and vegetables are grown, it is advantageous for temperature to be maintained in a range from about 18° C. to about 22° C. and relative humidity in a range from about 75% to about 82%.
In the closed environment of a greenhouse, RH tends to increase as a result of plant transpiration and evaporation of water from the soil and can be difficult to control. Typically, relative humidity in a greenhouse is controlled using a longstanding conventional procedure, in which hot humid air in the greenhouse is periodically vented to the outside environment and replaced with cooler air drawn into the greenhouse from the outside. The indrawn cool air is heated to bring its temperature within a desired range of greenhouse air temperatures. Heating the indrawn cool air also reduces its relative humidity. The capacity of air to hold water increases and its RH decreases with increasing air temperature. Relative humidity of indrawn cool air, even if it is 100% (i.e. at which relative humidity water begins to condense out of the air) may be reduced substantially by increasing that air's temperature. For example relative humidity of outside air at a temperature of 18° C. and 100% relative humidity is decreased to a relative humidity of 50% by heating to a temperature of 25° C.
Whereas the longstanding conventional procedure for controlling relative humidity by periodically venting hot humid greenhouse air and replacing it with cooler air drawn into the greenhouse from the outside and heated is generally effective, it exposes greenhouse plants to relatively large fluctuations in air temperature. The procedure also consumes relatively large amounts of energy and is therefore expensive.
By way of example, air temperature in a greenhouse using conventional humidity control systems may fluctuate from a low temperature equal to about an outside air temperature, for example, 10° C., to a maximum temperature of about 22° C. Relative humidity of the inside air may suffer a range from about 70% to about 100%. During a diurnal cycle for which outside relative humidity of outside air fluctuates between about 60% to about 70% and temperature of outside air between about 12° C. and 16° C. a conventional system may consume more than about 2,000 kWh (kilowatt hours) of energy.